The image was rendered on the CPU using an algorithm I've been trying to publish unsuccessfully for some time (I suspect due to some prior work on this topic). I call it Overnight Rendering with Bidirectional Path Tracing But actually the image looks pretty good already after 1 hour in my renderer which hasn't been scrutinized by a performance profiler this decade

The scene is attached below. It's in 3ds max 2012 format, with V-Ray materials. There are no explicit light sources in the scene, but two disks in the lap shades with emissive materials. I have a custom exporter for my renderer, and I also manually add a homogeneous medium with sigma_a = sigma_s = 0.1.

Indeed, the volume caustics are subtle but nice. Here's a visualization of length-4 paths only, where these caustics are more prominent. It'd be also cool to reproduce Veach's image pyramid with the contributions of the individual techniques.

There's actually built-in support for that in Mitsuba. All you need to do is to comment out/change a macro for it. It produces all the images for the pyramid, which can be easily put together with a script to form the actual pyramids (I've done that using an imagemagick script). Maybe it's easier to re-export it there than implementing the whole logic in your own renderer.